Shardfall

Head to Emaire to speak with the sagecrafter there. He will point you to the dungeon of Shardfall, which is full of ice sprites (bring fire) but first, bring him the heart of a Paragon Barghest to make you some kinda protective charm. Take the five minutes to locate the beast, rip its still beating heart out and dance in its bloody viscera before heading back to town.

The charm is optional - principally ice resistance. Prepare yourself and head into Shardfall (the dungeon, not the mission) and start cacking stuff. Waste the possessed mage, take the Froststone, and head back to town to collect.